Iridium-sheathed wire for electron tubes

ABSTRACT

A method of producing iridium coated tungsten or molybdenum wire, e.g. for use in grids of electron discharge tubes, comprises inserting into a tube of sintered iridium powder having a V.P.N. of from 220-310 a core of tungsten or molybdenum having a V.P.N. of 480/540 or 330/400 respectively and swaging the filled tube to rod and drawing the rod to wire.  In a first example iridium powder having a surface area of 0.3 m.2/gm. is compacted about a 0.25 inch diameter polished mild steel former under hydrostatic pressure of 15 tons/inch2. The former is flange at one end and the powder is supported by a latex sheath.  The compact 4 inches long and having 0.25 inch and 0.375 inch inside and outside diameters respectively is sintered in vacuum at 1500  DEG C. to give a density of 18 gms./cm.3. A sintered tungsten rod of a density of 15-16 gms./cm.3 is ground to produce a sliding fit and inserted into the compact, the assembly being subsequently swaged at 1500  DEG C. to 0.125 inch diameter rod, hot drawn at 750-650  DEG C. to 0.10 inch diameter wire, drawn at 600 DEG C. to 0.02 inch diameter and finally at 550-500  DEG C. to 0.01 inch diameter wire.  The drawing rate initially is from 1-2 ft./min. to about 12 ft./min. at a diameter of 0.02 inch.  The thickness of the iridium sheath is usually 0.001-0.002 inch but may be 0.0005 inch and the hardness of the iridium and tungsten 582 V.P.N. and 803 V.P.N. respectively.  Plugs of iridium may be used to plug the tube if the heating is carried out under oxidizing conditions.  In a second example a rod of swaged and drawn molybdenum containing 0.5% titanium and having a hardness of 375 V.P.N. is inserted in the tube, the ends plugged with iridium and the assembly hot-swaged and hot-drawn to a final diameter of 0.012 inch and a core diameter of 0.005 inch.  The iridium and molybdenum hardness values were 698 V.P.N. and 386 V.P.N. respectively.  The tensile strengths of the tube and core are preferably between 5000/12,000 p.s.i. and the density of the iridium, tungsten and molybdenum 16 to 20; 14 to 17 and 10.2 gms./cm.3 respectively.  It is stated that no fusing bond exists between the metal core and the iridium sheath which can be easily peeled therefrom.

United States Patent 3,271,849 IRIDlUM-SHEATHED ELECTRON TUBE Edward George Price, Chiswick, London, England, assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 8, 1963, Ser. No. 322,541 Claims priority, application Great Britain, Nov. 26, 1962, 44,639/ 62 3 Claims. (Cl. 29-4205) The present invention relates to wire for electron tubes and, more particularly, to iridium-sheathed wire having a core of tungsten or molybdenum.

Heretofore, the art has endeavored to provide wire suitable for use in high power electron tubes. The grid wire which is used in high power electron tubes must have low secondary-emission characteristics and must withstand temperatures up to 1500 C. resulting from electron bombardment. Secondary emission of grid wires is minimized by using materials with low emissivity. In the electronic industry, it is a known practice to make the grids of electron discharge tubes that are required to work at high temperature of molybdenum or tungsten wire coated with platinum. The platinum layer is capable of keeping the primary grid emission very low, a feature which is of particular importance in transmitting tubes, in which grids often assume a very high tempera ture. With the high power electron tubes now being made, the temperatures reached by the grid wires are such that volatilization of platinum occurs. Unfortunately, platinum has an appreciable vapor pressure and at temperatures of about 1500 C. and above a platinum-coated grid begins to lose platinum from its surface. Furthermore, in the case of grid wires made of platinum-coated molybdenum or platinum-coated tungsten operating at such elevated temperatures, a platinum-molybdenum or platinum-tungsten alloy which is electronically unsatisfactory is formed on the surface of the grid by diffusion.

Now it is known that iridium does not diffuse readily into molybdenum or tungsten, and it has a high work function (electron emission energy), good strength at high temperature and a lower vapor pressure than platinum. However as far as we are aware it has not hitherto been possible to produce an iridium-coated molybdenum or tungsten wire. It is notoriously difficult to attempt to deposit iridium by electroplating methods, and iridium is so refractory as to be very diflicult to work mechanically.

Although many attempts were made to overcome the foregoing difficulties and other difiiculties, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that sheathed wire having characteristics which render it highly suitable for use in high power electron tubes, which characteristics include a low work function at the wire surface, low volatilization and good strength at elevated temperatures of at least about 1500 C. and which also include resistance to having the sheath contaminated by diffusion with the core metal and freedom from foreign materials, can be produced by a new process involving powder metallurgy and hot working techniques.

It is an object of the present invention to provide wire having a low work function at the surface thereof, low v-olatilizati-on and good strength at elevated temperatures of at least about 1500 C. and low primary grid emission when used as a grid wire in a high power electron tube.

FOR

Another object of the invention is to provide iridiumsheathed wire having a core of tungsten or molybdenum which is free from diffusion between the metal of the sheath and the metal of the core and which is free from foreign substances.

It is also the purpose of the invention to provide a new process for producing wire having an iridium sheath and a core of tungsten or molybdenum without diffusion between the iridium and the tungsten or molybdenum and without interposition of a foreign substance between the sheath and the core.

Other objects and advantages will become apparent from the following description of the invention.

Generally speaking, the present invention is directed to production of iridium-sheathed wire having a core of tungsten or molybdenum without diffusion between the core metal and the iridium and without interposition of any foreign substance between the sheath and the core. Wire of the invention is made by hot swaging at about 1450 C. to about 1550 C. and hot drawing at about 500 C. to about 750 C. an assembly which is a filled tube comprising a special iridium tube and a tungsten or molybdenum rod having special characteristics which are correlated to characteristics of the iridium tube. Usually the iridium tube is of cylindrical shape with a concentric hole therethrough. The rod of core material is sized suitably for making a sliding lit with the hole in the tube. The outside and inside diameters of the sintered iridium tube for the assembly are in a ratio from 5:3 to 2:1. The swaged and drawn iridium sheath in the wire of the invention is a continuous penetration-resistant sheath that does not have discontinuities such as might be present if a sheath were applied by electroplating. The starting metal for the core of wire of the invention is a metal selected from the group consisting of sintered tungsten powder, swaged molybdenum containing up to about 0.5% titanium and swaged and drawn molybdenum containing up to about 0.5% titanium. Drawn tungsten is not satisfactory as a starting metal for making wire in accordance with the invention. It is highly advantageous that the metal of the core be characterized by a recrystallization temperature that is higher than the recrystallization temperature of the sheath metal in order that a fibrous structure be developed in the core.

According to the invention, iridium-sheathed tungsten or molybdenum wire is produced by inserting into a special tube of sintered iridium powder a core that makes a sliding fit in the tube, this core being a sintered rod of tungsten or a swaged or swaged and drawn rod of molybdenum, swaging the filled tube to rod and drawing down the swaged rod. It is essential that the resistances to deformation of the tube and the core be substantially the same. In accordance with the invention, the tensile strength of each is within the range of 5000 pounds per square inch (p.s.i.) to 12,000 p.s.i. If the resistances to deformation as shown by the tensile strengths are not substantially the same, excessive stresses are set up in the iridium tube and, as iridium is brittle in the sintered condition, the tube may and probably will fracture.

The sintered tube of iridium is made by compacting iridium powder and then sintering the compact in air or, preferably, in vacuum.

Considering tungsten before molybdenum, a discovery of the invention is that the desired'similar resistances to deformation are obtained by making the sintered density of the iridium from 16 grams per cubic centimeter (gms./

cm?) to 20 gms./cm. and the sintered density of the tungsten from 14 gms./cm. to 17 gms./cm. It is advantageous that if the sintered density of the iridium is near the lower end of its range, the sintered density of the tungsten should likewise be near the lower end. The corresponding hardnesses are from about 220 Vickers Diamond Penetration Number (V.P.N.) to about 310 V.'P.N. for the iridium and about 480 V.P.N. to about 520 V.P.N. for the tungsten.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention, the following illustrative examples are given. Advantageous methods of producing iridium-sheathed wire of the invention are further described in conjunction with the examples.

Iridium powder' having good sintering properties, that is to say, having a surface area of about 0.3 square meters per gram, was compacted around a polished mild steel former under a hydrostatic pressure of 15 long tons per square inch. The former, having a flange at one end, was surrounded by a latex sheath in which the powder was placed. The finished green compact was a tube of iridium of about 4 inches length, of 0.19 inch inside diameter, and of 0.37 inch outside diameter. The tub-e was then sintered in vacuum at 1500 C. The linear (lengthwise) shrinkage was about 16%, producing a tube having a sintered density of 18 gms./cm. a hardness of about 305 V.P.N., an outside diameter of 0.305 inch and an inside diameter of 0.18 inch.

A sintered tungsten rod of density of 15 gms./cm. to 16 gms./cm. and not above 17 gms./cm. and having a hardness of about 510 V.P.N. was ground to size to make a sliding fit within the sintered iridium tube. The filled tube was swaged at 1500" C. to /s inch diameter rod. Advantageously, the swage is one which will produce a smooth finish from fast-acting hammers and with small reductions a symmetrical sheath can be obtained. Iridium must be swaged quickly to ensure the metal temperature is high enough during the operation. The swaged rod was hot-drawn at 750 C. to 650 C. to wire of about 0.10 inch diameter. Thereafter, the wire was drawn at 600 C. down to 0.02 inch diameter and then at 550 C. to 500 C. to 0.01 inch diameter. The drawing rate varied from about 1 or 2 feet per minute (ft./ min.) at the start to about 12 ft./rnin. when the wire was of 0.02 inch diameter and smaller. The finished wire had an outside diameter of 0.01 inch, a core diameter of 0.0075 inch, a sheath hardness of 582 V.P.N. and a core hardness of 803 V.P.N.

When the tungsten core is placed within the iridium tube it can, if desired, be blocked at each end with plugs of iridium, but this is essential only if the heating operation is carried out under oxidizing conditions.

It is found that the sheath will tear unless at about 0.060 inch outside diameter the sheath has a thickness of at least 0.010 inch. As the outside diameter becomes smaller the force required to draw the wire is much reduced and the sheath can then come down in thickness.

The final wire can be of 0.01 inch diameter, the thickness of the iridium sheath being as low as 0.0005 inch, but usually from 0.001 to 0.002 inch. Thinner sheaths are obtained than would be expected from the initial diameter ratios of the iridium and the tungsten, which are between :3 and 2: 1, because in drawing from 0.03 inch to 0.01 inch diameter the iridium is deformed more than the tungsten.

Turning now to molybdenum, which as a metal is not as strong as tungsten, in accordance with the invention swaged molybdenum rod is used for the core in order to arrive at the essential similarity of resistance to deformation. When swaged or swaged and drawn molybdenum is the core metal, it is advantageous, in order to produce wire having an iridium sheath of good surface quality, that the molybdenum contain about 0.5% titanium. Such an alloy is characterized, among other advantageous characteristics, by a higher recrystallization temperature than pure molybdenum. For making molybdenum (including molybdenum containing up to 0.5 titanium) cored wire, the swaged and drawn condition with a slightly higher hardness (372 V.P.N.) than the swaged condition (337 V.P.N.) is the best starting condition. The iridium tube for molybdenum-cored wire is made in exactly the same way as when the core is tungsten. The ratio of the outer diameters of the tube and rod is advantageously about 2: 1. The rod has a tensile strength between 5000 and 12,000 p.s.i. and its density can be about 10.2 grams per cc.

An illustrative example of the production of wire having an iridium sheath and a core of an alloy of molybdenum containing about 0.5 titanium in accordance with the invention is as follows. A compacted and sintered iridium tube having the same characteristics and dimensions of the tube described in conjunction with the foregoing example of production of iridium-tungsten wire was produced by the same method of the foregoing example, except that the sintered tube of this example had an internal diameter of 0.15 inch. A rod of swaged and drawn molybdenum containing 0.5% titanium and having a hardness of about 375 V.P.N. was fitted into the tube, the rod being directed in contact with the iridium surface of the tube. The ends of the tube were sealed with iridium plugs to prevent oxidation. The filled and sealed tube was hot swaged and hot drawn to a final diameter of 0.012 inch in accordance with the methods employed for making the iridium-tungsten wire of the foregoing example. The final diameter of the molybdenum core was 0.005 inch. The sheath hardness of the finished wire was 698 V.P.N. and the core hardness was 386 V.P.N.

It is found that in the sheathed composite wire of the invention there is no bond between the core and the iridium, and in fact the outside sheath can be peeled away quite easily. This indicates that no diffusion has occurred between the core metal and the iridium, so that the wires are particularly suitable for use when diffusion between the two metals is undesirable from an electronic stand point.

The invention is particularly applicable to grid wire for electron tubes, especially for tubes in which the grid becomes as hot as 1500 C. or greater. Since the wire of the invention is essentially free of foreign substances, i.e., substances other than the aforementioned metals of the sheath and core, the wire is especially useful where it is desired to avoid having some such foreign substances in the enclosure within which the wire is used. The invention is also generally applicable where wire having oxidation resistance and high strength at elevated temperatures is required and especially where it is desired to have a sheathed wire that has high resistance to diffusion by the core metal.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for making iridium-sheathed wire especially adapted for use as grid wire in a high power electron tube comprising compacting and sintering iridium powder into a tube having outside and inside diameters in a ratio from 5:3 to 2:1, a Vickers Diamond Penetration hardness of 220 to 310 and a sintered density from 16 grams per cubic centimeter to 20 grams per cubic centimeter, providing a rod of metal characterized by tensile strength of 5000 pounds per square inch to 12,000 pounds per square inch and selected from the group consisting of sintered tungsten, swaged molybdenum containing up to about 0.5 titanium and swaged and drawn molybdenum containing up to about 0.5% titanium, fitting said metal rod into said iridium tube to make a filled tube, hot swaging said filled tube at about 1450" C. to about 1550 C and then hot drawing the filled tube at about 500 C. to about 750 C. to wire.

2. A process as set forth in claim '1 wherein the rod is of sintered tungsten having a density of 14 grams per cubic centimeter to 17 grams per cubic centimeter.

3'. A process as set forth in claim 1 wherein the rod is of swaged and drawn molybdenum containing up to about 0.5% titanium and is characterized by a recrystallization temperature which is higher than the recrystallization temperature of the iridium of the tube.

References Cited by the Examiner UNITED STATES PATENTS 5 2,334,609 11/1943 Cox 29420 2,373,405 4/1945 Lowit 29-420 2,628,516 2/1953 Brace 29-420 2,750,658 6/1956 Went 29420 10 HYLA'N-D BIZOT, Primary Examiner. 

1. A PROCESS FOR MAKING IRIDIUM-SHEATHED WIRE ESPECIALLY ADAPTED FOR USE AS GRID WIRE IN A HIGH POWER ELECTRON TUBE COMPRISING COMPACTING AND SINTERING IRIDIUM POWDER INTO A TUBE HAVING OUTSIDE AND INSIDE DIAMETERS IN A RATIO FROM 5:3 TO 2:1, A VICKERS DIAMOND PENETRATION HARDNESS OF 220 TO 310 AND A SINTERED DENSITY FROM 16 GRAMS PER CUBIC CENTIMETER TO 20 GRAMS PER CUBIC CENTIMETER, PROVIDING A ROD OF METAL CHARACTIZED BY TENSILE STRENGTH OF 5000 POUNDS PER SQUARE INCH TO 12,000 POUNDS PER SQUARE INCH AND SELECTED FROM THE GROUP CONSISTING OF SINTERED TUNGSTEN, SWAGED MOLYBDENUM CONTAINING UP TO ABOUT 0.5% TITANIUM, FITTING SAID METAL CONTAINING UP TO ABOUT 0.5% TITANIUM, FITTING SAID METAL ROD INTO SAID IRIDIUM TUBE TO MAKE A FILLED TUBE, HOT SWAGING SAID FILLED TUBE AT ABOUT TO MAKE A FILLED TUBE, HOT SWAGAND THEN HOT DRAWING THE FILLED TUBE AT ABOUT 500*C. TO ABOUT 175*C. TO WIRE. 